A valve device having a body forming a fluid flow path communicating with a combustion chamber in an internal-combustion engine and a valve supported pivotably in the body and driven by an actuator has been conventionally known. As such a valve device, a throttle valve device that can change an intake air mass fed to a combustion chamber, an EGR valve device that can change an exhaust gas mass recirculated from an exhaust path to an intake path, or the like is named.
Then as a valve device, a valve device of a W spring structure having a spring loading an open side urging force on a valve (hereunder referred to as an opening spring) and a spring loading a closed side urging force on a valve (hereunder referred to as a closing spring) is named (refer to Patent Document 1 for example).
In such a valve device, the urging force of an opening spring is added to a valve shaft when a valve is on the closed side of a predetermined intermediate position (a predetermined valve position set between a fully-closed position and a fully-open position) and the urging force of a closing spring is added to the valve shaft when the valve is located on the open side of the intermediate position. Then the valve device is structured so that the valve may stay at the intermediate position by the urging force of a spring in the state where the driving force of an actuator is zero (with regard to the detail of the structure, refer to Patent Document 1 for example).
More specifically, as illustrated in FIG. 10, a valve returns to an intermediate position only by a torque generated by the urging force of a closing spring when the driving force of an actuator is released in the region where the urging force of the closing spring is loaded (refer to the alternate long and two short dashes line with the arrow directed from a fully-open position toward the intermediate position in FIG. 10). Further, a valve returns to the intermediate position only by a torque generated by the urging force of an opening spring when the driving force of the actuator is released in the region where the urging force of the opening spring is loaded (refer to the alternate long and two short dashes line with the arrow directed from a fully-closed position toward the intermediate position in FIG. 10).
Here, in FIG. 10, the side of a rotary torque directed to the open side of a position where a load torque around a valve shaft is zero is represented by a positive side (upper side in the vertical axis) and the side of a rotary torque directed to the closed side is represented by a negative side (lower side in the vertical axis). In the present description, regardless of a positive side or a negative side, the magnitude of an absolute value is explained as the magnitude of a torque in a direction.
When a valve returns to an intermediate position by the release of the driving force of an actuator, a resistance force (frictional force and motor detent torque) is loaded on a valve shaft in the direction of restraining the valve from returning to the intermediate position by the urging force of a spring. As a result, the valve returns to the intermediate position by a torque generated by subtracting the resistance force from the urging force of the spring.
In this way, the set load of each of an opening spring and a closing spring is set at such a set load as to be able to return to an intermediate position only by an own urging force even when a resistance force exists. That is, the position where a valve can return from both an open side and a closed side unaidedly without the driving force of an actuator (self-return position) is an intermediate position.
In a valve device of such a structure, a torque exceeding the urging force of a closing spring and a resistance force (frictional force and the like) of hindering the rotation of a valve is required to be loaded by an actuator in order to move the valve from an intermediate position to an open side (refer to the thick solid line with the arrow directed from the intermediate position toward the fully-open position in FIG. 10). Further, a torque exceeding the urging force of an opening spring and a resistance force is required to be loaded by an actuator in order to move the valve from an intermediate position to a closed side (refer to the thick solid line with the arrow directed from the intermediate position toward the fully-closed position in FIG. 10).
Then an electric power supplied to an actuator increases as a required torque increases. In other words, an electric power supplied to an actuator increases as the urging force of a spring and a resistance force increase. In the case of a valve device that has to maintain a valve at a fully-closed position for a long period of time in particular, an actuator may burn out disadvantageously if the urging force of an opening spring and a resistance force are large. This is because a fully-closed position maintaining required torque necessary for maintaining a valve at a fully-closed position increases. For the reason, there is a need for reducing the urging force of an opening spring and restraining the burnout of an actuator.
When the urging force of an opening spring is reduced however, a valve cannot return to a predetermined intermediate position only by the urging force of the opening spring and stops disadvantageously at a position on the closed side of the intermediate position.
When an intermediate position is set as a position allowing a valve to be restrained from freezing and sticking, the valve may freeze and stick disadvantageously if the valve can return only to a position on the closed side of the intermediate position. Here, the freezing and sticking of a valve means the phenomenon of the valve being immovable because moisture included in an intake gas or an exhaust gas in a fluid flow path condenses under a low temperature environment after an IG switch (ignition switch) is turned off and the moisture freezes between the valve and a body.
For the reason, the situation of stopping a valve at a position on the closed side of an intermediate position after an IG switch is turned off is required to be avoided to the utmost extent. In other words, a valve is required to return to an intermediate position without fail after an IG switch is turned off even when the valve is located on the closed side of the intermediate position at the time of commanding the IG switch to turn off.
Consequently, there is a request for locating a valve position after an IG switch is turned off at an intermediate position to the utmost extent in addition to a request for restraining the burnout of an actuator and a structure capable of materializing both the requests is needed.